Have you had the tires high speed balanced?
I believe I heard someone else mention that in a post a while back.
On another note, My Friends dad had a 600SEL that would vibrate over 60. in 1994, he had to invoke lemon law. he drove out with a new one after he had taken the car in 7 times over 2 years.
Now this may not apply to you, but I would definately get your suspension components checked out for various things. The bushings in the suspension might be worn.
My old tire shop, which is no longer around, used to skim the tires with a machine. I think that They used to do it to remove the high spots on the tires in the out of balance areas. it would not just sit againstt he tire skimming it down, it would do it in burps when the wheel reached certain out of balance area's. I am not sure what the exact purpose of the tire skimming is for, but I would assume its to make the tires more perfectly round and or flat in the center area. If someone can confirm this, it might help if it hasn't been done.
I would think its either related to the wheels/tires or the suspension.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.